Conventionally manufactured friction materials are produced by mechanically blending particulate friction-producing and lubricating additives, short fibers and thermosetting resin materials and then molding the mass under heat and pressure to form a finished brake pad or lining or the like. During molding of the mixed material, pressure is applied perpendicular to the broad face of the pad. This results in a high degree of fiber orientation generally parallel to what is to be the wear surface of the pad. This procedure and the resulting fiber orientation limits the friction and wear performance of the final pad due to material restriction and the inability to orient fibers in the optimal direction, which would often be perpendicular to the wear surface.
One of the primary limitations of friction materials is their thermal stability. On a conventional automotive disk brake pad, the wear surface temperature can exceed 700.degree. C. The wear rate of the surface materials increases dramatically as the wear surface temperature rises above the decomposition temperatures of the fibers and/or the binder that joins them together. The fibrous constituents serve as both a reinforcement for the friction material and a bearing surface to increase wear life. Thus, it is desirable to incorporate fibers into the friction material with the decomposition temperature above 700.degree. C. Such fibers include metallic, carbon, graphite, glassy and ceramic fibers. Except for the metallic fibers, all of these classes of potential friction material reinforcing fibers are intrinsically brittle, and they fracture with a loss of reinforcing capability when subjected to the mixing and molding pressures used to form friction materials by conventional processes.
Thus, conventional friction material processing does not allow the exploitation of the most thermally-resistant fibers due to breakage of the fibers and to their alignment in a least desirable orientation parallel to the wear surface. When the fibers are in line with the contact of the wear surface, they are readily pulled from the matrix when the binder wears away and exposes the length of the fiber. When the fiber is gone, there is additional binder wear and so forth. Thus, in the conventional friction material processing, the high temperature-resistant brittle fibers break and they are aligned in a direction not especially suitable to the integrity of the friction material.
In addition to the above problems, the conventional mixing of friction material formulations often lends itself to segregation of the materials so that the friction material is not uniform in its mixture, as would be most desirable.
Accordingly, it is apparent that alternative and improved practices for the making of friction materials would be useful. It is an object of the present invention to provide such a method.